1. Field of the Invention
This invention relates generally to thermochromic materials and, in particular, to thermochromic pigments which are reversible color pigments that change from a dark color to colorless in the presence of heat or thermal energy, and then darken to the original color as the pigment is cooled.
2. Background of the Invention
In the past thermochromic color changing materials or pigments have generally been used in paints, inks, dyes and dyed fabrics. Thermochromic pigments have also been used in toys, dolls, recording media and novelties such as seals on beverage mugs that change color when a hot or cold beverage is placed in the mug.
For example U.S. Pat. No. 4,681,791 to Shibahashi et.al. describes a textile material in the form of fiber, raw stock, yarn or fabric comprising a plurality of fibers coated with a thermochromic layer containing a binder and a thermochromic pigment which includes an electron-donating, color former, an electron-accepting developer and a color changing controlling agent. Other prior art patents which disclose color changing compounds include U.S. Pat. No. 4,851,282 to Shimizu et al. which discloses material comprising a core covered with a thermochromic vinyl chloride resin sol composition which undergoes a reversible color change upon detecting a change in temperature and U.S. Pat. No. 4,931,221 to Heller which discloses photochromic benzopyran and naphthopyran spiran compounds which change color when exposed to light radiation involving ultraviolet rays.
A number of these prior art thermochromic pigments or materials are formed using an organic dye, an acid activator and a low melting solid such as ester or alcohol that acts as a solvent when liquified. The thermochromic mixtures of the prior art are generally dark or colored when cool or at room temperature, but becomes decolorized when heated or warmed to the melting point of the solid solvent.
The acid activators used in these prior art color changing pigments or materials include phenols, carboxylic acids, phosphinic and phosphonic acids and metal salts of these acids. Although these acid activators are quite effective in causing reversible color changes in thermochromic pigments or thermobleaching of the thermochromic dyes, these activators gradually deteriorate the organic dyes used in prior art color changing pigments causing an irreversible color change.
With the disadvantages inherent in prior art color changing pigments and materials when used, for example, in textile materials, in paints and coatings and as inks, the reversible thermochromic pigments constituting the present invention were conceived and one object of the invention is to provide novel pigment compositions that change color reversibly from a dark color when cold or at room temperature, to colorless when heated or hot at a specific controlled temperature.
Another object of the present invention is to provide a pigment composition of reversible color which can be incorporated into paints, coatings, inks, dyes and textile materials to eliminate the above described limitations including the elimination of irreversible color change in the thermochromic pigments.
Yet another object of the present invention is to provide a pigment composition which can be microencapsulated in an alkaline media, the acid catalyzed composition of the prior art.
Various other advantages and objectives of the present invention will become apparent to those skilled in the art by the detailed description of the invention and preferred embodiments.